1. Field of the Invention
The subject invention is directed to devices for measuring pressures, especially high pressures, and temperatures in a hydraulic or pneumatic system. More specifically, the subject invention concerns devices that incorporate both a piezo-resistive pressure measuring cell and an electronic circuit in one housing to form an integral component.
2. Description of the Prior Art
The usual types of manometers known in the prior art are generally read off only at intervals. Furthermore, such devices are frequently inadequate with regard to the precision required for certain measurements. In contrast, electronic measuring devices afford a continuous recording of the measured values and a rapid processing of the signals with a high degree of measuring accuracy.
Hydraulic systems are becoming increasingly complicated in the course of the rapid technical development. One consequence of this is that the hydraulic systems are placing increasingly higher demands on the required accuracy and switching speed of associated systems. As a result, there is a demand for improved pressure and temperature measuring devices that will not only continuously indicate the actual values measured by the electronic circuit, but will also give an instantaneous or realtime indication, of interference in the installation.
In piezo-resistive pressure measuring cells known in the prior art, the piezo-resistive semiconductor element (silicium chip) that is used for recording the pressure is connected to the corresponding contacts by means of binding wires. Thus, the entire sensor housing is standardized from semiconductor technology and is similar to that used for transistors, operational amplifiers, and the like. Consequently, such prior art sensor housings are unsuitable for measuring high pressures because the pressure influence on the housing distorts the measurement. Moreover, the housing often cannot withstand the higher pressures that are to be measured such that it is subject to deformation.
Auslegeschrift No. 26 30 640 describes a piezo-resistive pressure measuring cell known in the prior art. The cell therein described is located inside an oil-filled transferring membrane housing. Base pins are embedded in molded glass recesses that extend along the periphery of the transfer membrane housing. The pressure measuring cell unit is bonded onto a small base wafer that has a bigger diameter than the cell itself. In order to lead into the base pins, inlets are provided along the periphery which correspond to the base pins of the transfer membrane housing. The base pins lead the electrical contacts of the sensor out of the pressure measuring cell to the electronic circuitry that is extraneous to the system and that serves to supply the sensor system and to amplify the generated signals. Thus, a compact measuring system that can be connected to a pressurized hydraulic or pneumatic system by a simple threaded and measuring adapter cannot be formed.
In particular, for measuring high pressures and for measuring pressures under extreme conditions such as high temperatures, adjusting devices are required to take precise measurements in every range. However, in order to adjust the pressure sensor and to continuously maintain it during the measurements, it is necessary to measure and evaluate the physical parameters prevailing in the immediate environment of the pressure sensor that influence the measuring accuracy of the pressure sensor.
One property of the piezo-resistive measuring cell, which is made from a Si-chip, is that it normally delivers a positive or a negative voltage at pressures of zero bar. In order to use such a measuring cell for a precise pressure measurement, it is necessary to provide an adjustment. Therefore, it is especially important that the measuring cell, with an actual prevailing pressure of zero bar, also precisely indicates a zero bar pressure with zero output voltage.
The voltage delivered from the Si-chip measuring cell is relatively low and therefore has to be amplified. In the known Si-chip and the electrical connections are effected by means of plug-in or threaded contacts. Thus, the signals, and particularly the low-voltage signals, are subject to interference. Such interference must be avoided in order to make a precise measurement.
Thus, there was a need in the prior art for a pressure measuring device capable of measuring high pressures that would avoid interference by measuring the physical parameters that have an adverse effect on the pressure sensor. If possible, such parameters would also be evaluated at the location where they influence the sensor. In this way, these values are not also subject to interference.
In addition, there was a need for such a device that was sturdy, that could readily be connected to hydraulic or pneumatic systems, and that would indicate an exceptionally accurate value with a system pressure of zero bar.